# Direct-initialization

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< cpp‎ | language

Initializes an object from explicit set of constructor arguments.

### Syntax

T object `(` arg `);` T object `(` arg1, arg2, ... `);` | (1) |
T object `{` arg `};` | (2) | (since C++11)
T `(` other `)` T `(` arg1, arg2, ... `)` | (3) |
`static_cast<` T `>(` other `)` | (4) |
`new` T`(` args, ... `)` | (5) |
Class`::`Class`()` `:` member`(` args, ... `)` `{` ... `}` | (6) |
`[`arg`]() {` ... `}` | (7) | (since C++11)

### Explanation

Direct-initialization is performed in the following situations:

1) Initialization with a nonempty parenthesized list of expressions or braced-init-lists(since C++11).
2) Initialization of an object of non-class type with a single brace-enclosed initializer (note: for class types and other uses of braced-init-list, see list-initialization)(since C++11).
3) Initialization of a prvalue temporary(until C++17)the result object of a prvalue(since C++17) by function-style cast or with a parenthesized expression list.
4) Initialization of a prvalue temporary(until C++17)the result object of a prvalue(since C++17) by a static_cast expression.
5) Initialization of an object with dynamic storage duration by a new-expression with an initializer.
6) Initialization of a base or a non-static member by constructor initializer list.
7) Initialization of closure object members from the variables caught by copy in a lambda-expression.

The effects of direct-initialization are:

- If `T` is an array type,

the program is ill-formed. | (until C++20)
the array is initialized as in aggregate initialization, except that narrowing conversions are allowed and any elements without an initializer are value-initialized.
    struct A
    {
        explicit A(int i = 0) {}
    };

    A a[2](A(1)); // OK: initializes a[0] with A(1) and a[1] with A()
    A b[2]{A(1)}; // error: implicit copy-list-initialization of b[1]
                  //        from {} selected explicit constructor
| (since C++20)

- If `T` is a class type,

if the initializer is a prvalue expression whose type is the same class as `T` (ignoring cv-qualification), the initializer expression itself, rather than a temporary materialized from it, is used to initialize the destination object. (Before C++17, the compiler may elide the construction from the prvalue temporary in this case, but the appropriate constructor must still be accessible: see copy elision) | (since C++17)

  - the constructors of `T` are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.

otherwise, if the destination type is a (possibly cv-qualified) aggregate class, it is initialized as described in aggregate initialization except that narrowing conversions are permitted, designated initializers are not allowed, a temporary bound to a reference does not have its lifetime extended, there is no brace elision, and any elements without an initializer are value-initialized.
    struct B
    {
        int a;
        int&& r;
    };

    int f();
    int n = 10;

    B b1{1, f()};            // OK, lifetime is extended
    B b2(1, f());            // well-formed, but dangling reference
    B b3{1.0, 1};            // error: narrowing conversion
    B b4(1.0, 1);            // well-formed, but dangling reference
    B b5(1.0, std::move(n)); // OK
| (since C++20)

- Otherwise, if `T` is a non-class type but the source type is a class type, the conversion functions of the source type and its base classes, if any, are examined and the best match is selected by overload resolution. The selected user-defined conversion is then used to convert the initializer expression into the object being initialized.
- Otherwise, if `T` is bool and the source type is std::nullptr_t, the value of the initialized object is false.
- Otherwise, standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version of `T`, and the initial value of the object being initialized is the (possibly converted) value.

### Notes

Direct-initialization is more permissive than copy-initialization: copy-initialization only considers non-explicit constructors and non-explicit user-defined conversion functions, while direct-initialization considers all constructors and all user-defined conversion functions.

In case of ambiguity between a variable declaration using the direct-initialization syntax (1) (with round parentheses) and a function declaration, the compiler always chooses function declaration. This disambiguation rule is sometimes counter-intuitive and has been called the most vexing parse.

Run this code

    #include <fstream>
    #include <iterator>
    #include <string>

    int main()
    {
        std::ifstream file("data.txt");

        // The following is a function declaration:
        std::string foo1(std::istreambuf_iterator<char>(file),
                         std::istreambuf_iterator<char>());
        // It declares a function called foo1, whose return type is std::string,
        // first parameter has type std::istreambuf_iterator<char> and the name "file",
        // second parameter has no name and has type std::istreambuf_iterator<char>(),
        // which is rewritten to function pointer type std::istreambuf_iterator<char>(*)()

        // Pre-C++11 fix (to declare a variable) - add extra parentheses around one
        // of the arguments:
        std::string str1((std::istreambuf_iterator<char>(file)),
                          std::istreambuf_iterator<char>());

        // Post-C++11 fix (to declare a variable) - use list-initialization for any
        // of the arguments:
        std::string str2(std::istreambuf_iterator<char>{file}, {});
    }

### Example

Run this code

    #include <iostream>
    #include <memory>
    #include <string>

    struct Foo
    {
        int mem;
        explicit Foo(int n) : mem(n) {}
    };

    int main()
    {
        std::string s1("test"); // constructor from const char*
        std::string s2(10, 'a');

        std::unique_ptr<int> p(new int(1));  // OK: explicit constructors allowed
    //  std::unique_ptr<int> p = new int(1); // error: constructor is explicit

        Foo f(2); // f is direct-initialized:
                  // constructor parameter n is copy-initialized from the rvalue 2
                  // f.mem is direct-initialized from the parameter n
    //  Foo f2 = 2; // error: constructor is explicit

        std::cout << s1 << ' ' << s2 << ' ' << *p << ' ' << f.mem  << '\n';
    }

Output:

    test aaaaaaaaaa 1 2

### See also

- copy elision
- converting constructor
- copy assignment
- copy constructor
- default constructor
- destructor
- `explicit`
- initialization
  - aggregate initialization
  - constant initialization
  - copy initialization
  - default initialization
  - initializer list
  - list initialization
  - reference initialization
  - value initialization
  - zero initialization
- move assignment
- move constructor
- `new`

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